Permitted emissions from internal combustion engines, such as diesel engines, are legislated by governments. Amongst the legislated exhaust gas species are nitrogen oxides (NOx), carbon monoxide (CO), hydrocarbons (HC) and particulate matter (PM). The levels of permitted emissions of these species are being progressively reduced over the next 10 to 15 years. Original equipment manufacturers (OEMs) are seeking to meet these legislated requirements through a combination of engine design and exhaust gas aftertreatment.
In order to meet existing and future legislated requirements for diesel PM, one device proposed for exhaust gas aftertreatment is the particulate filter. By “filter” herein we mean devices that remove a solid particle from an exhaust gas and also devices that intentionally delay the progress of the particle through the exhaust system. An example of the latter group of devices is described in EP 1057519 (incorporated herein by reference).
One example of a particulate filter is a wall-flow filter in which the filter is in the form of a honeycomb. The honeycomb has an inlet end and an outlet end, and a plurality of cells extending from the inlet end to the outlet end, the cells having porous walls wherein part of the total number of cells at the inlet end are plugged, e.g. to a depth of about 5 to 20 mm, along a portion of their lengths, and the remaining part of the cells that are open at the inlet end are plugged at the outlet end along a portion of their lengths, so that a flowing exhaust gas stream passing through the cells of the honeycomb from the inlet end flows into the open cells, through the cell walls, and out of the filter through the open cells at the outlet end. A composition for plugging the cells is described in U.S. Pat. No. 4,329,162 (incorporated herein by reference). A typical arrangement is to have every other cell on a given face plugged, as in a chequered pattern.
It is known to catalyse such filters in order to lower the soot combustion temperature to facilitate regeneration of the filter passively by oxidation of PM under exhaust temperatures experienced during regular operation of the engine/vehicle, typically in the 300-400° C. range. In the absence of the catalyst, PM can be oxidized at appreciable rates at temperatures in excess of 500° C., which are rarely seen in diesel engines during real-life operation. Such catalysed filters are often called catalysed soot filters (or CSFs).
A common problem with passive filter regeneration is that driving conditions can prevent exhaust gas temperatures achieving even the lower temperatures facilitated by catalysing the filter frequently enough to reliably prevent PM from building up on the filter. Such driving conditions include extended periods of engine idling or slow urban driving and the problem is particularly acute for exhaust gas from light-duty diesel engines. One solution to this problem which has been adopted by OEMs is to use active techniques to regenerate the filter either at regular intervals or when a predetermined filter backpressure is detected in addition to passive regeneration. A typical arrangement in a light-duty diesel vehicle is to position a diesel oxidation catalyst (DOC) on a separate monolith upstream of the CSF and to regulate in-cylinder fuel combustion by various engine management techniques in order to introduce increased amounts of unburned fuel into the exhaust gas. The additional fuel is combusted on the DOC, increasing the temperature in the downstream CSF sufficiently to promote combustion of PM thereon.
EP-A-0341382 or U.S. Pat. No. 4,902,487 (both incorporated herein by reference) describes a method of treating diesel exhaust gas including PM and NOx unfiltered over an oxidation catalyst to convert NO to NO2, collecting the PM on a filter downstream of the oxidation catalyst and combusting trapped PM in the NO2. This technology is commercially available as Johnson Matthey's CRT®. An advantage of this process is that the combustion of PM in NO2 occurs at temperatures of up to 400° C., i.e. closer to the normal operating window for diesel exhaust gases, whereas combustion of PM in oxygen occurs at 550-600° C.
Our WO 01/12320 (incorporated herein by reference) describes a reactor especially suitable for treating exhaust gases to remove pollutants including PM and comprises a wall-flow filter structure with porous walls and alternate blocked ends, wherein a washcoat carrying a catalyst coats a zone at an upstream end of open channels at an upstream end of the filter.
We have now devised a CSF for use in a passive-active filter regeneration regime that makes more efficient use of the more limited exhaust gas temperatures and variation of exhaust gas temperatures from diesel engines, especially light-duty diesel engines.
According to one aspect the invention provides a diesel engine comprising an exhaust system, which exhaust system comprising: a particulate filter made from a porous material having a mean pore diameter of from 5 μm to 40 μm, and a porosity of at least 40%, e.g. 50% to 70%, and a bulk volumetric heat capacity of at least 0.50 J cm−3 K−1 at 500° C., which filter comprising a diesel oxidation catalyst (DOC) located in a zone on the front end of the filter for oxidising carbon monoxide (CO), hydrocarbons (HC) and nitrogen monoxide (NO), the engine comprising engine management means, in use, to provide continuously or intermittently an exhaust gas comprising sufficient nitrogen oxides (NOx) or HC and/or an exhaust gas of sufficiently high temperature to combust particulate matter (PM) in the filter.
The catalysed soot filter of the present invention combines a number of very useful functions: it collects soot from the exhaust gas; it promotes passive oxidation of the collected soot in oxygen; it promotes NO oxidation to promote the passive combustion of collected soot in NO2 according to the process described in EP 0341832; and it converts CO and HC in the exhaust gas at relatively low temperatures. Additionally, by positioning the DOC on the front of the filter, active regeneration of the filter is promoted, because the exotherm from combusting additional HC contributes to heating the filter directly, i.e. there is no temperature loss between an upstream DOC and the downstream CSF. Accordingly, active regeneration is more efficient, requiring less fuel to raise the filter temperature to effect regeneration.